1. Field
The invention relates to harvesting of crops that are planted in rows, such as corn and soybeans. A harvesting vehicle is steered automatically by sensing the locations of the rows of plants. Typically, corn is planted with a predetermined spacing of from 6 inches to 14 inches, in rows about 21/2 feet apart. When the plants are harvested, a vehicle travels in a direction longitudinal of the rows of corn, automatically senses the lateral locations of at least one of the rows, and utilizes the information for fully-automatic steering or operator-assisted automatic steering.
Reference is made to a co-pending application of the same inventors and presently assigned to the same assignee as this application. It is: "Automatic Steering Apparatus For Crop Vehicle", Ser. No. 302,889, filed Jan. 30, 1989, now U.S. Pat. No. 4,967,362.
2. Background
Crop vehicles of this type commonly harvest twelve rows of plants simultaneously. As the vehicle is driven forward the plants in the rows are guided by twelve V-shaped guides at the front of the vehicle. The V-shaped guides shepherd the flexible plants into stripping or cutting bars mounted on the crop vehicle, where the ears of corn are stripped from their stalks or the plants are severed. The V-shaped guides are typically about five feet long, and their forward tips are about 21/2 feet apart. The front of at least one of the V-guides is equipped with a pair of plant-contacting sensors. These feeler-type sensors are touched by the corn plants as they enter the V-guide. The contact sensors provide electrical signals that assist in the steering of the crop vehicle.
An example of only one of many prior art systems is one that can automatically assist the steering well enough to harvest corn satisfactorily from straight rows of plants, but is unable to steer very accurately where the rows are curved. Typical requirements include being able to track down to a 50-meter turning radius, at 5 mph, with a 20"-wide interception "window" for each row of plants. For steering purposes that particular prior art system relies upon information as to the magnitude of deflection of each of two plant-contacting sensors (a right-hand sensor and a left-hand sensor, at the front of the same V-guide).
The reason that the prior vehicle does not automatically follow curved rows of plants very well is that a relatively large error (i.e., lateral offset of the vehicle from the optimum travel path) has to develop before a sufficient corrective steering command is produced. This defect cannot be corrected by merely designing the closed feedback loop with higher gain and providing an integral feedback term in the steering control system because the system then becomes unstable.
To improve its steering, the prior art system referred to above employs a bias signal that is controlled manually. When the vehicle enters a turn the operator is required to turn a knob to add enough bias signal to keep the crop vehicle lined up with the rows of crop. Needless to say it is inconvenient for the operator to have to assist the automatic steering system in this way.